With the excellent research of Hirumi et al. (1), techniques have been developed for the growth of infective forms of African trypanosomes in vitro. The system involves the presence of tissue cells over which are placed bloodstream trypomastigotes. We have been able to successfully grow and maintain T. rhodesiense over several types of commercially available tissue cells, including Chinese hamster lung cells (ATCC CCL 16 Don) (2). We have also grown cultures of two strains of T. rhodesiense (EATRO 1895) and (WRATat) on New Zealand white rabbit kidney, Rhesus monkey kidney, and bovine embryonic trachea tissue cells, all available from ATCC. In our initial studies, the trypanosomes were grown using 20% fetal bovine serum. However, it is now possible to substitute either 10% newborn calf serum with the EBTr cells or 10% horse serum with all the tissue cells. We are also investigating the biochemical properties of the trypanosomes cultured in vitro. It is clear that the predominant terminal oxidase is the alpha-glycerophosphate oxidase system. All other biochemical parameters examined suggest these organisms are identical to bloodstream trypomastigotes. We can also use tissue cells irradiated at 5000 rads. After one year, the trypanosomes remain infective to mice, although the time before death is extended significantly the longer the cultures are grown in vitro, suggesting an attenuation of the strains. No invasive or cytopathic effects on the tissue cells have been observed. Using clones of T. rhodesiense WRATat strains of a particular antigenic type and hybridoma produced specific antisera, we have been trying to detect different antigenic types after growth of the trypanosomes in culture, in order to support the suggestion that the mechanism for antigenic variation does not require the presence of the host immune system.